adding expiration date to an autoresponder

I have a program (SendStudioNX) that collects subscribers in an sql database via a php script. When emails are sent out to this list they need to have an expiration date since they are coupons as well. I need to have a custom field that runs as "expiration date" which will always be 14 days from the day the email was sent out. I would like to base the 14 days from the server-side so that I am not relying on the customer's computer's date to be right.